1. Field of the Invention
The present invention relates to a process for obtaining improved glass envelopes for electric light sources and to the glass envelopes obtained by the process.
2. Description of the Prior Art
The inner surfaces of bulbs for electric light sources such as incandescent lamps, high and low pressure mercury lamps etc. are in many cases covered with light refracting, coloring or luminescent powders, with transparent enamels or with heat reflecting layers depending on the desired properties. Numerous processes are known for preparing such coatings. The wet process is the most widely used.
There has been a desire to substitute the wet process by a so-called dry process, since such dry processes provide numerous advantages when compared with the wet process.
A major advantage of the dry process is to be seen in providing coatings free from binders requiring removal after coating. Among the most important dry processes are the electrostatic processes. Numerous workers have tried to find a suitable electrostatic process and several processes have been developed.
S. A. Lopenski et al in U.S. Pat. No. 2,811,131 issued Oct. 29, 1957 discloses a process for depositing electrostatically a fine silica layer (aerosil) on the inner walls of lamp bulbs. This process has the disadvantage of providing inadequate adhesion of the deposited layer and requires improvement of the adhesion of this layer on the inner surface of the bulb by an after-treatment consisting of an evaporation process followed by a heat-treatment.
West-German Pat. (DE-AS) No. 1,064,698 discloses a process for depositing electrostatically silica powder containing titanium dioxide on the inner wall of a light bulb. This process improves the optical parameters of the bulbs compared to those obtained by earlier processes employing pure silica, but it does not solve the fundamental problem.
West-German Pat. (DE-PS) No. 1,078,688 discloses an electrostatic process for coating of glass envelopes such as employed in incandescent lamps. This process is suitable for depositing silica in finely-divided form, however, the problem of providing an adhering coating could only be solved by applying a heat treatment at high temperature.
British Patent Specification No. 827,013 discloses another process for increasing the adhesion of the deposited silica powder. The patent provides for an addition of for example 0.5% TiO.sub.2 of high density, but the mechanism of this process remains unexplained.
As can be recognized from the above references there has been a considerable effort in the past to improve the preparation of electrostatic coatings. All conventional processes had the drawback of leaving unresolved the problem of adhesion of the deposited pigments. The adhesion could only be obtained by employing for example an after-treatment at elevated temperature resulting in degradation of the optical parameters of the coating and thereby limiting severely the applicability of such process.